For Paul Stamets, the phrase mushroom hunt does not denote a leisurely stroll with a napkin-lined basket. This morning, a half-dozen of us are struggling to keep up with the mycologist. He points to a clutch of plump oyster mushrooms halfway up an alder trunk. These could clean up oil spills all over the planet, he says. He ducks beneath a rotting log, where a rare, beehive-like Agarikon dangles. This could provide a defense against weaponized smallpox. He plucks a tiny, gray Mycena alcalinafrom the soil and holds it under our noses. Smell that? It seems to be outgassing chlorine. To Stamets, that suggests it can break down toxic chlorine-based polychlorinated biphenyls, or PCBs. Most Americans think of mushrooms as ingredients in soup or intruders on a well-tended lawn. Stamets, however, cherishes a grander vision, one trumpeted in the subtitle of his 2005 book, Mycelium Running: How Mushrooms Can Help Save the World. Mushroom-producing fungi, he believes, can serve as game changers in fields as disparate as medicine, forestry, pesticides and pollution control. He describes mycelium, the web of fibrous tissue from which mushrooms spring, as the neurological network of nature, a sentient membrane that has the long-term health of the host environment in mind. To some, such language seems uncomfortably metaphysical. Yet Stamets ideas have gained an expanding audience among mainstream scientists, environmental engineers, federal officials and Silicon Valley investors.